quicktime settings advanced

I had a problem with installing 5kplayer (probably malware?) Stupid me! I could get rid of it with Cleanmymac. But now Quicktime doen't work: QTX crashes and QT7 plays the sound but doesn't show the movie. Strangely enough iTunes works and iMovie also. Any suggestions?

Hi, Working intensively in iMovie recently exposed a very strange quirk. It might be user error on my side but still, I would like to know if any of you have ever faced it. I "clipped" my project's timeline with chapter markers across the whole of its length. If I select to export with...